Heterogeneously integrated photonic devices with improved optical coupling between waveguides

ABSTRACT

An optical device comprises first, second and third elements fabricated on a common substrate. The first element comprises an active waveguide structure supporting a first optical mode, the second element, fabricated on a planarized top surface of the first element, comprises a passive waveguide structure supporting a second optical mode, and the third element, at least partly butt-coupled to the first element, comprises an intermediate waveguide structure, positioned such that a top surface of the intermediate structure underlies a bottom surface of the passive waveguide structure. If the first optical mode differs from the second optical mode by more than a predetermined amount, a tapered waveguide structure in at least one of the second and third elements facilitates efficient adiabatic transformation between the first optical mode and the second optical mode. Mutual alignments of the first, second and third elements are defined using lithographic alignment marks.

FIELD OF THE INVENTION

The present invention relates to semiconductor processing. Morespecifically, certain embodiments of the invention relate to a methodand system for realization of photonic integrated circuits usingdissimilar materials that are optically coupled.

BACKGROUND OF THE INVENTION

A photonic integrated circuit (PIC) or integrated optical circuit is adevice that integrates multiple photonic functions and as such isanalogous to an electronic integrated circuit. The major differencebetween the two is that a photonic integrated circuit provides functionsfor information signals imposed on optical carrier waves.

PICs can be realized in a number of material systems, generallydetermined by the wavelength of operation, such as those based on indiumphosphide (InP), gallium arsenide (GaAs), or gallium nitride (GaN) whichare direct bandgap materials capable of efficient light generation.Other PICs can be realized by indirect bandgap materials such as e.g.silicon (Si) or dielectrics such as e.g. silicon nitride (SiNx),aluminum oxide (Al2O3), lithium niobate (LiNbO3) or others. Often, asingle material system does not provide complete functionality (lowpropagation loss, high or low confinement in the waveguide, electricallypumped optical gain, efficient modulation, etc.) or parts of thatfunctionality provide sub-optimal performance. The materials ofdifferent material systems can differ in temperature ofgrowth/deposition, thermal expansion coefficients and/or otherparameters.

This problem is generally solved by assembling PICs comprising two ormore chips made from dissimilar materials in separate processes. Such anapproach is challenging due to a need for very fine alignment, whichincreases packaging costs and introduces scaling limitations. Anotherapproach to solving the problem is to bond dissimilar materials andprocess them together, removing the need for precise alignment andallowing for mass fabrication. In this disclosure, we use the term“hybrid” to describe the first approach that includes precise assemblyof separately processed parts, and we use the term “heterogeneous” todescribe the latter with no precise alignment necessary.

Standard heterogeneous integration is very powerful, but might bechallenging in some materials combinations due to a variety of reasonsincluding small native wafers (limiting the total available material, asin e.g. native GaN), problems related to bonding surface quality (e.g.thick buffer layer growth resulting in increased surface roughness),problems relating to substrate removal (e.g. absence of appropriatelyselective etches) and/or others.

A second limitation of standard heterogeneous integration arises from alimitation in the smallest feature size that can readily be fabricated.To transfer the optical signal between dissimilar materials, theheterogeneous approach utilizes tapers whose dimensions are graduallyreduced until the effective mode refractive indices of dissimilarmaterials match and there is efficient power transfer. This approachgenerally works well when materials have similar refractive indices asis the case with silicon and InP. In cases where there is a largerdifference in effective indices, such as between e.g. SiN and GaAs, therequirements on taper tip dimensions become prohibitive, limitingefficient power transfer. Specifically, extremely small taper tip widths(of the order of nanometers) may be necessary to provide good coupling.Achieving such dimensions is complex and may be cost prohibitive.

There remains, therefore, a need for methods that provide efficientoptical coupling between materials with dissimilar refractive indices,without requiring prohibitively narrow taper tips, and that can beachieved using a fabrication process capable of handling large wafers.This would allow for scalable integration of various materials for therealization of PICs. Ideally, PICs made by such a method would operateover a wide wavelength range from ultra-violet (UV) to infra-red (IR)and be able to handle the high optical powers needed for manyapplications.

SUMMARY OF THE INVENTION

The present invention includes devices and methods for providingpractical and efficient optical coupling between elements comprisingmaterials of different refractive indices, with particular relevance tointegrated PICs.

In one embodiment, an optical device comprises first, second and thirdelements fabricated on a common substrate; wherein the first elementcomprises an active waveguide structure supporting a first optical mode,the second element, fabricated on a planarized top surface of the firstelement, comprises a passive waveguide structure supporting a secondoptical mode, and the third element, at least partly butt coupled to thefirst element, comprises an intermediate waveguide structure, positionedrelative to the passive waveguide structure such that a top surface ofthe intermediate structure underlies a bottom surface of the passivewaveguide structure.

In this embodiment, if the first optical mode differs from the secondoptical mode by more than a predetermined amount, a tapered waveguidestructure in at least one of the second and third elements facilitatesefficient adiabatic transformation between the first optical mode andthe second optical mode. Mutual alignments of the first, second andthird elements are defined using lithographic alignment marks.

In another embodiment, a method for making an optical device comprises:forming a first element, comprising an active material, on a substrate;defining an active waveguide, configured to support a first opticalmode, in the first element; and patterning a surface in the firstelement; forming an intermediate element and defining an intermediatewaveguide supporting an intermediate optical mode in the intermediateelement; planarizing a top surface of the first element; forming asecond element on the planarized top surface; defining a passivewaveguide configured to support a second optical mode in the secondelement, and positioned such that a top surface of the intermediatewaveguide underlies a bottom surface of the passive waveguide; andforming electrical contacts in the first element. The intermediatewaveguide in the intermediate element is tapered to facilitatetransformation between the first and second optical modes if the firstoptical mode differs from the second optical mode by more than apredetermined amount. Relative positionings of the first, intermediate,and second and third elements are defined using lithographic alignmentmarks.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a device according to one embodiment of the presentinvention, shown in cross-section.

FIG. 2 illustrates two devices according to two correspondingembodiments of the present invention, shown in top-down view.

FIG. 3 illustrates two devices according to two correspondingembodiments of the present invention, shown in cross-section.

FIG. 4 illustrates a device according to one embodiment of the presentinvention, shown in cross-section.

FIG. 5 illustrates a process flow diagram of a method according to someembodiments of the present invention.

DETAILED DESCRIPTION

Described herein include embodiments of a method and system forrealization of photonic integrated circuits using epitaxial growth,wafer bonding and/or deposition of dissimilar materials where opticalcoupling is improved by use of mode conversion and a butt-couplingscheme.

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, wherein like numeralsdesignate like parts throughout, and in which are shown by way ofillustration embodiments in which the subject matter of the presentdisclosure may be practiced. It is to be understood that otherembodiments may be utilized and structural or logical changes may bemade without departing from the scope of the present disclosure.Therefore, the following detailed description is not to be taken in alimiting sense, and the scope of embodiments is defined by the appendedclaims and their equivalents.

The description may use perspective-based descriptions such astop/bottom, in/out, over/under, and the like. Such descriptions aremerely used to facilitate the discussion and are not intended torestrict the application of embodiments described herein to anyparticular orientation. The description may use the phrases “in anembodiment,” or “in embodiments,” which may each refer to one or more ofthe same or different embodiments. Furthermore, the terms “comprising,”“including,” “having,” and the like, as used with respect to embodimentsof the present disclosure, are synonymous.

For the purposes of the present disclosure, the phrase “A and/or B”means (A), (B), or (A and B). For the purposes of the presentdisclosure, the phrase “A, B, and/or C” means (A), (B), (C), (A and B),(A and C), (B and C), or (A, B and C).

The term “coupled with,” along with its derivatives, may be used herein.“Coupled” may mean one or more of the following. “Coupled” may mean thattwo or more elements are in direct physical, electrical, or opticalcontact. However, “coupled” may also mean that two or more elementsindirectly contact each other, but yet still cooperate or interact witheach other, and may mean that one or more other elements are coupled orconnected between the elements that are said to be coupled with eachother. The term “directly coupled” means that two or more elements arein direct contact in at least part of their surfaces. The term“butt-coupled” is used herein in its normal sense of meaning an “end-on”or axial coupling, where there is minimal or zero axial offset betweenthe elements in question. The axial offset may be, for example, slightlygreater than zero in cases where a thin intervening layer of some sortis formed between the elements, as described below with regard to e.g.elements 240 and 290 in FIG. 2. It should be noted that the axes of twowaveguide structures or elements need not be colinear for them to beaccurately described as being butt-coupled. In other words, theinterface between the elements need not be perpendicular to either axis.Device 250, shown in FIG. 2 discussed below, is one example of suchpossibilities.

The terms “active device” and/or “active region”, may be used herein. Adevice or a region of a device called active is capable of lightgeneration, amplification, modulation and/or detection. We use activedevice and active region interchangeably meaning either one of themand/or both. This is in contrast to “passive device” and/or “passiveregion” whose principal function is to confine and guide light, and orprovide splitting, combining, filtering and/or other functionalitiesthat are commonly associated with passive devices. Some passive devicescan provide functions overlapping with active device functionality, suchas e.g. phase tuning implemented using thermal effects or similar thatcan provide modulation. The difference in this case is in performance,with active devices generally providing higher efficiencies, lower powerconsumption, larger bandwidth and/or other benefits. No absolutedistinction should be assumed between “active” and “passive” basedpurely on material composition or device structure. A silicon device,for example, may be considered active under certain conditions ofmodulation, or detection of short wavelength radiation, but passive inmost other situations.

FIG. 1 is a schematic cross-section view of an integrated photonicdevice 100 utilizing butt-coupling and mode conversion for efficientcoupling between dissimilar materials. The device includes a substrate105 that can be any suitable substrate for semiconductor epitaxialgrowth and/or integration, such as Si, InP, GaAs, GaN, quartz,silicon-on-insulator, sapphire or other materials known in the art. Suchintegration, aside from direct epitaxial growth, can include, but is notlimited, to bonding of semiconductor material to substrate and/orbonding of template on which subsequently semiconductor material isgrown. Substrate 105 can comprise multiple layers providing additionalfunctionality such as buffer layers typically used to improve thequality of grown films and reduced density of defects. Substrate 105 canalso comprise various dielectric or semiconductor layers providingoptical cladding functionality, optical filtering and/or reflectionfunctionality (typically realized as multilayer structures as is knownto one skilled in the art) and/or others. In some embodiments, substrate105 is transparent in the operating wavelength range, e.g. quartz whenthe PIC operates in visible or longer wavelength range.

Layer 110 is epitaxially grown, bonded, otherwise deposited and/ortransferred on top of substrate 105 and can be made up of materialsincluding, but not limited to, InP and InP-based ternary and quaternarymaterials, GaAs and GaAs based ternary and quaternary materials, GaNbased ternary and quaternary materials, GaP, InAs and InSb and theirvariations and derivatives. Layer 110 in one embodiment is multilayered,comprising layers providing both optical and electrical confinement aswell as electrical contacts, as is known in the art for active devices.In yet another embodiment, layer 110 uses substrate 105 to provideelectrical and/or optical confinement and one or more electricalcontacts. In one embodiment, materials grown in layer 110 can be of thesame (native) material system to substrate 105 (e.g. GaN substrate andGaN based active layer) or they can be grown on different materialsystems (e.g. sapphire substrate and GaN based active layer). Growth onnon-native systems can be beneficial economically (e.g. lower costwafers and/or larger wafers in different material system such as e.g. Sior sapphire), can provide additional functionality (e.g. transparency)and/or have other benefits. Layer 110 is processed to form opticaland/or electrical confinement and other elements needed for defining anactive structure as is commonly done in the art. In some embodiments,frequency selective surfaces such as e.g. gratings are formed. Suchstructures in one embodiment are used to define single frequency laserssuch as distributed feedback (DFB) or distributed Bragg reflector (DBR)lasers. The processing includes standard techniques such asphotolithography and/or other methods of patterning, dry and wet etches,chemical-mechanical polishing (CMP), depositions and/or othertechniques. Layer 110 can be completely removed in some parts of thestructure as shown in FIG. 1 or parts of layer 110 can be preserved (notshown).

Layer 115 is deposited and/or grown, after the deposition/growth andpatterning of layer 110, using techniques known in the field. In oneembodiment, the material of layer 115 may include, but is not limitedto, one or more of SiN, TiO2, Ta2O5, SiO2, and AlN. In some embodiments,other common dielectric materials may be used for layer 115. In otherembodiments, a semiconductor material is used for layer 115. In oneembodiment, the effective refractive index of material comprising layer115 is lower than that of layer 130 (to be described later). Due to thepatterning of layer 110, prior to deposition/growth of layer 115, in oneembodiment, the deposited or grown layer 115 has a non-planar uppersurface, the non-planarity of which is reflective of the non-planarityof the surface on which 115 is deposited or grown, a juxtaposition ofunderlying layers 105 and 110.

After layer 115 has been deposited or grown, the top surface of one orboth of layers 115 and 110 may be planarized by chemical mechanicalpolishing (CMP) or other etching, chemical and/or mechanical polishingmethods. In one embodiment, also shown in FIG. 1, the planarizationcompletely removes any parts of the layer 115 that remain on top oflayer 110. In other embodiments (not shown), a thin extension of layer115 may be left on top of layer 110 after such polishing. In otherembodiments, the smooth, even planarity of the top surface of layer 115occurs automatically because of the intrinsic nature of the method bywhich layer 115 is deposited, for example if the material of layer 115is a spin-on glass, polymer, photoresist or other suitable material. Theterm “planarized” is defined herein as applying equally to surfaces thatare intrinsically smooth at a level typically achieved with aplanarization step (e.g. CMP processes with <5 nm of local RMSroughness, where variations might be larger at wafer scale level) orhave been polished by means as described above to render them smooth tothat level.

Layer 115 is patterned and removed in a region into which layer 130 issubsequently deposited. Whether or not planarization of the top surfaceof layer 115 is achieved before the deposition of layer 130, it isimportant that planarization is achieved after that deposition, so thatlayer 120 (to be described below) has a smooth underlying interface.Layer 130 in one embodiment has higher refractive index than layer 115(e.g. layer 130 can be silicon-oxynitride while layer 115 can besilicon-dioxide or other combinations of suitable materials). Layer 130is butt-coupled to layer 110 either directly, or with a thin interveninglayer between 130 and 110, providing coating functionality (not shown inFIG. 1 but explained in relation to FIG. 2 and elements 240 and/or 290).The coating typically provides high-reflectivity, anti-reflection and/orpassivation functionality. The interface between 130 and 110 can beangled (meaning at an angle other than 90 degrees relative to awaveguide axis) to further optimize reflection as described below withrespect to FIG. 2, which shows one such case. The top surface of layer130 may be intrinsically planarized or may be deliberately planarized,as discussed above regarding layer 115. In either of these two cases,layer 115 has a planarized top surface¹, whereas layer 130 may have ormay not have a planarized surface. ¹ Whether performed in one step(after the deposition/growth of layer 130) or in two steps (before andafter the deposition/growth of layer 130)

Layer 120 is deposited on top of a planar top surface that includes theplanar top surface of layer 115 but may also include one or both ofplanar top surfaces of layers 130 and 110. In one of the embodiments,the refractive index of layer 120 is larger than the refractive index oflayer 115. Layer 120 can be pattered to provide waveguiding and/or othercommon functionality implemented in passive devices such as powersplitting, wavelength splitting, filtering, tuning (e.g. thermal),adjusting the mode shape for e.g. fiber coupling and/or others.

The upper cladding layer 125 for waveguides realized in 120 and/or 130can be ambient air (meaning no cladding material is actually deposited)or can be any other deliberately deposited suitable material as shown inFIG. 1, including, but not limited to, a polymer, SiO2, SiNx, etc. Layer125 can also provide cladding functionality to layer 110 and/or supportthe deposition of electrical contacts 135. In the embodiments shown inFIG. 1, only one contact is shown, but it is clear that devicesgenerally have one or more contacts that can be realized on a topsurface as shown in FIG. 1 but may also involve using the substrateand/or any other suitable architecture, as would be well known in thefield.

Layer 130 serves as an intermediate waveguide that in some embodimentsaccepts the profile (depicted by dashed line 151) of an optical modesupported by the waveguide primarily defined in layer 110, captures itefficiently as mode profile 152, and gradually transfers it to modeprofiles 153, and finally 154. Mode profile 154 is then efficientlycoupled to the waveguide for which layer 120 provides the core. In otherembodiments, the direction of travel may be reversed, with layer 130efficiently transforming optical mode 154, supported by the waveguidefor which layer 120 provides the core, to that of a mode 151, supportedby the waveguide primarily defined in layer 110. The refractive index oflayer 130 can be engineered to facilitate efficient coupling of modeprofile 151, via a butt-coupling mechanism, and to efficiently transformthe mode to one with mode profile 154, by taking the advantage oftapered structures made in layer 120 and/or 130. Prior to the presentinvention i.e. in the absence of intermediate layer 130, therequirements on taper tip width for direct transfer from mode 151 to 154would be problematic. The use of intermediate layer 130, however,significantly reduces the stringent requirements on taper tip width,allowing efficient transfer between very high refractive index materials(such as e.g. GaAs or GaN in layer 110) to low refractive indexmaterials (such as e.g. SiN, SiO2 in layer 120). Yet another advantageof this coupling scheme is the ability to efficiently capture modes 151whose center is significantly offset in vertical direction from thecenter of mode 154, which is very challenging utilizing only taperedstructures.

It is to be understood that optical coupling between modes in layer 110and layer 120 is reciprocal, so that, taking FIG. 1 as exemplary, thestructure can be configured to facilitate light transmission from region110 to region 120 as explicitly shown, but also to facilitatetransmission in the reverse direction, from region 120 to region 110. Itis to be understood that multiple such transitions with no limitation intheir number or orientation can be realized on a suitably configuredPIC.

FIG. 2 depicts a top-down view of a device 200 according to oneembodiment of the present invention where boundaries between dissimilarmaterials are perpendicular to the direction of propagation and atop-down view of a device 250 according to one embodiment of the presentinvention where boundaries between dissimilar materials are angled(non-perpendicular) relative to propagation directions, to control boththe transmission and back reflection. One or more lithography alignmentmarks 245/295 are present to facilitate precise alignment between thelayers formed during various processing steps.

Functional layers 205/255 correspond to functional layer 105 in FIG. 1.Functional layers 210/260 correspond to functional layer 110 in FIG. 1.Functional layers 220/270 correspond to functional layer 120 in FIG. 1.Functional layers 230/280 correspond to functional layer 130 in FIG. 1.

In the embodiment shown in 200, there is an optional layer 240 depositedat the interface between 210 and 230. Optional layer 240 (and optionallayer 290 in device 250) primarily serves as either an anti-reflectiveor a highly reflective coating at the interface between layer 210 andlayer 230. Layer 240 can also serve as a passivation layer for thefacet/sidewall of layer 210. Layer 230 serves as an intermediatewaveguide that facilitates efficient coupling between modes supported bywaveguide whose core is formed by layer 220 and waveguide formed inlayer 210 as explained with the help of FIG. 1. The dimensions of layer220 can be tapered as illustrated with a taper tip 221. In otherembodiments, the width of 230 can be tapered while width of 220 is keptconstant. In yet another embodiment, both widths (of 220 and 230) can beadjusted to facilitate more efficient optical coupling. The requirementson the dimensions of the taper tips are relaxed with proper choice ofmaterials allowing for mass fabrication using standard lithographytools. Tapers might terminate before the actual interface (as shown ine.g. 200 where 221 terminates before the interface with layer 240), canterminate at interface (not shown) and/or extend past the interface (notshown).

In the embodiment shown in 250 one or more of the interfaces betweenlayers 260, 290, 280, and/or 270 are angled to reduce corresponding backreflection(s). The angle 295 defines the angle between the normal to thedirection of propagation of the wave inside structure 260 and thefacet/interface with layer 290 (and/or 280 if layer 290 is not present).In some embodiments the angle varies between 1° and 45°. In someembodiments, the angle is substantially equal to 8°.

The angle 296 defines the angle between the direction of the propagationof the wave inside the structure 260 and the angle of the waveguideformed by 280. The angle is an optimization parameter for couplingefficiency and is related to the choice of the angle 295, refractiveindices of the materials used and/or other parameters. In one embodimentit is substantially equal to 0°. In yet another embodiment it is between1° and 45°. In yet another embodiment it is substantially equal to 8°.

The magnitude of any lateral offset or misalignment between the axisdefined by the direction of the propagation of the wave inside thestructure 260 and the center of the waveguide 280 at the interface to260, and/or 290 is an optimization parameter where such offset can bepositive (in an “upward” direction as viewed in FIG. 2), negative (downin FIG. 2) and/or substantially equal to 0 (no offset). Suchoptimization is straightforward to perform with numerical software tomaximize the performance of the transition.

FIG. 3 shows two schematic cross-section views of some embodiments of anintegrated photonic device utilizing butt-coupling and mode conversionfor efficient coupling between dissimilar materials. Functional layers305/355 correspond to functional layer 105 in FIG. 1. Functional layers310/360 correspond to functional layer 110 in FIG. 1. Functional layers315/365 correspond to functional layer 115 in FIG. 1. Functional layers325/375 correspond to functional layer 125 in FIG. 1. Functional layers335/385 correspond to functional layer 135 in FIG. 1.

In one embodiment, an integrated photonic device 300 deposits only layer320 that serves dual functionality: (1) to provide efficientbutt-coupling of optical modes supported by layer 310 to optical modessupported by waveguides defined in layer 320 (effectively providing thefunctionality of layer 130 in FIG. 1) and (2) provide all the passivefunctionality provided by equivalent layer 120 in FIG. 1. This dualfunctionality is provided with no intermediate deposited layers.

In yet another embodiment, an integrated photonic device 350 depositsboth layers 370 (corresponding to functional layer 120 in FIG. 1) and380 (corresponding to functional layer 130 in FIG. 1) such that theirtop surfaces are aligned and planarized in a single planarization step.Their top surfaces can also be aligned with top surface of layer 360 (asshown in 360) or can be at a different height than the top surface of360 (not shown). In such a case (not shown), either one or both oflayers 370 and/or 380 cover at least parts of layer 360 top surface.Layer 370, as in the shown embodiment, can terminate at the interfacewith layer 380. In another unshown embodiment, layer 370 may extendinside the layer 380, as mentioned above with respect to taperembodiments related to the shown FIG. 2 embodiment.

FIG. 4 is a schematic cross-section view of an integrated photonicdevice 400 utilizing butt-coupling and mode conversion for efficientcoupling between dissimilar materials supporting integration of two ormore active materials, with an intervening passive waveguide structure.

Consider the right-hand side of the figure first. Functional layer 405corresponds to functional layer 105 in FIG. 1. Functional layer 410corresponds to functional layer 110 in FIG. 1. Functional layer 415corresponds to functional layer 115 in FIG. 1. Functional layer 420corresponds to functional layer 120 in FIG. 1. Functional layer 430corresponds to functional layer 130 in FIG. 1. This part of device 400therefore enables efficient coupling between waveguides 410 and 420 inthe same way as device 100 enables efficient coupling between waveguides110 and 120.

Now consider the left-hand side of the figure. The layer structure shownhere is similar to structures described in U.S. Pat. No. 1,071,889. Itbegins with one or more active layers 445, in this case bonded to thetop surface of layer 420, after layer 420 has been deposited andpatterned. The bonding can be direct molecular bonding or can useadditional materials to facilitate bonding such as e.g. metal layers orpolymer films as is known in the art. Typical materials from which layer445 is made include, but are not limited to, InP and InP-based ternaryand quaternary materials, GaAs and GaAs based ternary and quaternarymaterials, GaN, GaP, InAs and InSb and their variations and derivatives.

Layer 445 in one embodiment is multilayered, comprising sub-layersproviding both optical and electrical confinement as well as electricalcontacts, as is known in the art for active devices. The sub-layersgenerally provide vertical confinement. In yet another embodiment, layer445 uses lower layers 415 and/or 420 to provide electrical and/oroptical confinement and one or more electrical contacts. Horizontalconfinement, in one of the embodiments, is provided by defining a ridgetype structure into layer 445. Horizontal confinement can also beprovided by generating a strip structure, by implants or othertechniques in the field. In either case, the intent of confinementstructures is to control the position and shape of the optical mode andoptimize the interaction between the optical mode and injected,generated and/or depleted carriers.

Layer 435 serves as an intermediate waveguide that facilitates efficientcoupling between modes supported by waveguide whose core is formed bylayer 420 and waveguide formed in layer 445 similarly to coupling fromlayer 110 to layer 120 as explained with the help of FIG. 1. Therefore,embodiments of the type shown in FIG. 4 enable efficient coupling notonly between waveguides formed in layer 410 and layer 420 but alsobetween waveguides formed in layer 420 and layer 445.

In some embodiments, layer 445 can be efficiently electrically pumped togenerate optical emission and gain. Efficient coupling is facilitated bylayer 435. An optional layer (not shown) can serve as an anti-reflectiveor a highly-reflective coating at the interface between layer 445 andlayer 435 (similarly to layers 240/290 described in relation to FIG. 2).The optional layer can also serve as a passivation layer for thefacet/sidewall of layer 445,

Layer 435 may comprise a dielectric, a polymer and/or any other suitablematerial. The upper cladding layer 425 for waveguides realized in 420,430 and/or 435 can be ambient air (meaning no cladding material isactually deposited) or can be any other deliberately deposited suitablematerial including, but not limited to, a polymer, SiO2, SiNx, etc.Layer 425 can also support electrical contacts 440 a and/or 440 bdeposition. In the embodiments shown in FIG. 4, only one contact peractive region is shown, but it is clear that devices generally have oneor more contacts that can be realized on a top surface as shown in FIG.4 but may also involve using the substrate and/or any other suitablearchitecture, as would be well known in the field.

The dimensions of layer 420 can be tapered similarly as shown in FIG. 2and illustrated with a taper tip 221. In certain embodiments, one orboth of the waveguides defined in layers 420 and 435 are tapered (notshown). The requirements on the dimensions of the taper tips are relaxedwith proper choice of materials allowing for mass fabrication usingstandard lithography tools.

FIG. 5 is a process flow diagram of a method according to someembodiments of the present invention, showing some of the operationscarried out to make integrated devices of the types described above withrespect to FIGS. 1 and 2, and a set of cross-sectional views of acorresponding integrated photonic device at certain points duringfabrication.

Method 500 for making the devices need not always include all thefunctions, operations, or actions shown, or to include them in exactlythe sequence illustrated by the sequence from blocks 505 through 540 asshown. In an exemplary case, however, to provide devices such as thosediscussed above with reference to FIG. 1, method 500 begins with block,505, in which a substrate, suitably prepared for subsequent processingsteps, is provided. Method 500 may then proceed from block 505 to block510, where a first element, comprising one or more semiconductormaterial layers, is formed on the prepared substrate. The first elementcan be formed by an epitaxial growth technique such as metal organicchemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) or someother growth technique known in the art, by bonding, or be otherwisedeposited and/or transferred onto the top of the substrate. Across-section illustrating one embodiment at block 510 of the method isshown by 555.

From block 510, method 500 may proceed to block 515 where waveguides andother optical components such as, but not limited to, detectors,amplifiers, optical sources and/or others are formed in first element bypatterning, etching, deposition, and/or other techniques used insemiconductor processing. Block 515 can also include definition of metalcontacts and other steps typically performed when processing activeoptical devices. During block 515, layer 115 is created. Layer 115, as apart of the first element, can provide various functionalities includingoptical confinement, surface passivation, thermal resistanceoptimization and/or others, but also serves to provide claddingfunctionality to the second element (to be described later).

From block 515, method 500 may proceed to block 520 in which the topsurface of layer 115 may optionally be planarized. As discussed above,this may be achieved by depositing a layer of material on top of thepartially fabricated device, and then, if the method of deposition doesnot intrinsically provide a planar top surface, carrying out adeliberate planarization process such as CMP. The planarization may becontrolled to leave a layer of desired, typically very low, thickness ontop of the active waveguide layer in the first element, or to remove allof the deposited material above the level of the top surface of thefirst element. A cross-section illustrating one embodiment at block 520of the method is shown by 560.

The method proceeds to block 525, in which an intermediate element isformed over or above the substrate, typically by removing/etching partsin which a layer of an additional material (layer 130 in FIG. 1) isdeposited. A cross-section illustrating one embodiment at block 525 ofthe method is shown by 565.

At block 527, planarization of at least one top surface in preparationfor the deposition or growth of a second element is ensured. This mayhave already been partly achieved by the nature of the deposition methodfor layer 115, or by a deliberate planarization effort (such as CMP)after forming that layer, but in either case, the top surface of layer115 at this stage of process 500 is planarized. In addition, the topsurfaces of one or both layers corresponding to 130 and 110 in FIG. 1may optionally be planarized.

The method proceeds to block 530, in which a second element is formedabove the substrate, typically by depositing a layer (120 in FIG. 1) ofan additional material as is known in the art. This layer, in someembodiments, can also be bonded onto a top surface of the first element.After forming the second element layer, the second element can bepatterned to define waveguides and other optional structures, such as,but not limited to, couplers, filters, resonators, etc. A cross-sectionillustrating one embodiment at block 530 of the method is shown by 570.

The method proceeds to block 535, in which a top cladding is formedtypically by depositing a layer of an additional material as is known inthe art. A cross-section illustrating one embodiment at block 535 of themethod is shown by 575.

Next, at step 540, electrical contacts may be formed/finalized. Thisstep may itself contain multiple processing steps including etching,deposition, lift-off and/or others. In embodiments where the firstelement is used to provide a semiconductor light source and/oramplifier, these contacts are used to drive the light source to generatelight. In embodiments where the first element is used to provide aphotodetector, these contacts may be used to convey the photodetectoroutput signals. A cross-section illustrating one embodiment at block 540of the method is shown by 580.

Further processing of the various dielectric and/or semiconductorlayers, and/or electrical contacts, and the addition and processing ofindex matching layers, upper cladding, bonding pads, etc may beperformed as is known in the art.

Embodiments of the present invention offer many benefits. Theintegration platform enables scalable manufacturing of PICs made frommultiple materials and capable of covering a wide wavelength range fromvisible to IR and handling high optical power compared to typical Siwaveguide-based or InP waveguide-based PICs.

Previous approaches have generally used taper structures in order totransfer an optical mode from an active device to a passive device,where a width of compound semiconductor region is adiabatically tapereddown to sub-micron size. However, a required width of the taper tipdecreases rapidly to tens of nanometer sizes as the difference inrefractive indices increases. The present invention deploys a buttcoupling scheme to eliminate the need of a very small taper size in thecompound semiconductor waveguide, which eases fabrication of suchstructures.

Other approaches have relied on die attachment of prefabricated opticalactive devices to passive waveguides. This requires very stringentalignment accuracy which is typically beyond what a typical die-bondercan provide. This aspect limits the throughput of this process as wellas the performance of optical coupling.

This present invention utilizes a process flow consisting of epitaxialgrowth and typically wafer-bonding of a blanket piece of compoundsemiconductor and/or dielectric material on a carrier wafer andsubsequent semiconductor fabrication processes as is known in the art.It enables an accurate definition of optical alignment between activeand passive waveguides typically via a photo lithography step, removingthe need for precise physical alignment. Photo lithography-basedalignment allows for scalable manufacturing using wafer scaletechniques.

In some embodiments the active region can utilize the substrate for moreefficient thermal sinking, due to direct contact to the substrate withno dielectric in-between. In such embodiments, active region fullydefines the optical waveguide in active region and transitions topassive region via the above-mentioned butt-coupling.

Embodiments of the optical devices described herein may be incorporatedinto various other devices and systems including, but not limited to,various computing and/or consumer electronic devices/appliances,communication systems, sensors and sensing systems.

It is to be understood that the disclosure teaches just few examples ofthe illustrative embodiment and that many variations of the inventioncan easily be devised by those skilled in the art after reading thisdisclosure and that the scope of the present invention is to bedetermined by the following claims.

The invention claimed is:
 1. A device comprising: first, second andthird elements fabricated on a common substrate; wherein the firstelement comprises an active waveguide structure supporting a firstoptical mode, the second element, fabricated on a planarized top surfaceof the first element, comprises a passive waveguide structure supportinga second optical mode, and the third element, at least partlybutt-coupled to the first element, comprises an intermediate waveguidestructure, positioned relative to the passive waveguide structure suchthat a top surface of the intermediate structure underlies a bottomsurface of the passive waveguide structure; wherein a high reflectivitycoating layer is present between the active waveguide structure and theintermediate waveguide structure; wherein, if the first optical modediffers from the second optical mode by more than a predeterminedamount, a tapered waveguide structure in at least one of the second andthird elements facilitates efficient adiabatic transformation betweenthe first optical mode and the second optical mode; and wherein mutualalignments of the first, second and third elements are defined usinglithographic alignment marks.
 2. A device comprising: first, second andthird elements fabricated on a common, substrate, top surfaces of thefirst, second and third elements being co-planar; wherein the firstelement comprises an active waveguide structure supporting a firstoptical mode, the second element, fabricated on a planarized top surfaceof the first element, comprises a passive waveguide structure supportinga second optical mode, and the third element, at least partlybutt-coupled to the first element, comprises an intermediate waveguidestructure, positioned such that a top planarized surface of theintermediate waveguide structure is aligned with a top planarizedsurface of the passive waveguide structure; wherein, if the firstoptical mode differs from the second optical mode by more than apredetermined amount, a tapered waveguide structure in at least one ofthe second and third elements facilitates efficient adiabatictransformation between the first optical mode and the second opticalmode; and wherein mutual alignments of the first, second and thirdelements are defined using lithographic alignment marks.
 3. The deviceof claim 2, wherein the butt-coupled interface between the activewaveguide structure and the intermediate waveguide structure is angled.4. The device of claim 2, wherein the active waveguide structure in thefirst element comprises an optical source.
 5. The device of claim 2,wherein the active waveguide structure in the first element comprises aphotodetector.
 6. A device comprising: first, second, third, fourth andfifth elements fabricated on a common substrate; wherein the firstelement comprises an active waveguide structure supporting a firstoptical mode, the second element, fabricated on a planarized top surfaceof the first element, comprises a passive waveguide structure supportinga second optical mode, and the third element, at least partlybutt-coupled to the first element, comprises an intermediate waveguidestructure, positioned relative to the passive waveguide structure suchthat a top surface of the intermediate structure underlies a bottomsurface of the passive waveguide structure; wherein, if the firstoptical mode differs from the second optical mode by more than apredetermined amount, a tapered waveguide structure in at least one ofthe second and third elements facilitates efficient adiabatictransformation between the first optical mode and the second opticalmode; wherein the fourth element comprises a second active waveguidestructure supporting a fourth optical mode, and the fifth element, atleast partly butt-coupled to the fourth element, comprises a secondintermediate waveguide structure; wherein, if the fourth optical modediffers from the second optical mode by more than a predeterminedamount, a tapered waveguide structure in at least one of the second andfifth elements facilitates efficient adiabatic transformation betweenthe fourth optical mode and the second optical mode; and wherein mutualalignments of the first, second, third, fourth and fifth elements aredefined using lithographic alignment marks.
 7. The device of claim 6,wherein at least one of the butt-coupled interface between the activewaveguide structure and the intermediate waveguide structure, and thebutt-coupled interface between second active waveguide structure and thesecond intermediate waveguide structure is angled.
 8. The device ofclaim 6, wherein the active waveguide structure in the first elementcomprises an optical source.
 9. The device of claim 6, wherein theactive waveguide structure in the first element comprises aphotodetector.